The present invention relates to a color image forming method for use in electrophotography, electrostatic recording, electrostatic printing, and similar techniques.
There are known a transfer drum type and photoreceptor body color superimposition type as color image forming methods for electrophotography.
In the transfer drum type, first an electrostatic image on a photoreceptor drum is developed, for example, with a yellow toner, and the developed image is transferred to image carrying material (transfer paper) wound on a transfer drum. Such a process is repeated for yellow, magenta, and cyan toners and a black toner as needed, thereby forming a full color image (see the Japanese Patent Application Laid-Open Nos. 42-23910, 43-24748, 60-76766, and 64-15774.)
However, the transfer drum type electrophotography heretofore used has the following disadvantages.
(1) The apparatus has to be made large as it needs the transfer drum for holding the image carrying material.
(2) A complicated arrangement is needed to hold on or release from the transfer drum the image carrying material.
(3) It is necessary to provide an arrangement and control system to precisely register the photoreceptor body and the transfer drum.
On the other hand, in the photoreceptor drum color superimposition type electrophotography, the color toner images of the yellow, magenta, and cyan toners are superimposed to form the image, and these are all transferred at one time onto image carrying material (transfer paper). The color superimposition type includes a single composition developer type (see the Japanese Patent Application Laid-Open Nos. 1-283574, 2-46474, and 2-55368) and a two composition developer type (see the Japanese Patent Application Laid-Open Nos. 47-27537, 59-58452, and 1-193763). These are desirable in some fields because they need no transfer drum so that the whole apparatus can be made small.
The color superimposition on the photoreceptor body is proceeded as follows.
(1) The whole surface of the photoreceptor body, as shown in FIG. 1(a), is uniformly charged to a potential of -V.sub.H (1st charge).
(2) The first color exposure is made as shown in FIG. 1(b) (1st exposure). This exposure decreases the surface potential on the photoreceptor body to V.sub.L. The potential V.sub.B shown in FIG. 1(b) is a bias potential.
(3) The first toner is inversion developed as shown in FIG. 1(c) (1st development). This increases the surface potential of the developing portions by the toner layer potential of -V.sub.1 due to the negative charge the toner has. That is, the potential of -V.sub.L before the development becomes -(VL+V1) after it.
(4) The whole surface of the photoreceptor body, as shown in FIG. 1(d), is uniformly charged for the second color process (2nd charge).
(5) The second color exposure is made for superimposition of the second color as shown in FIG. 1(E) (2nd exposure). This exposure makes the surface potential on the photoreceptor body to -(V.sub.L +V.sub.1) because of the toner layer potential of the first color toner if its light intensity is same as in the 1st exposure.
(6) The second toner is inversion developed as with the first color as shown in FIG. 1(f) (2nd development). This increases the surface potential of the developing section by the toner layer potential of -V.sub.2 due to the negative charge the second toner has. That is, the potential of -(V.sub.L +V.sub.1) before the development becomes -(V.sub.L +V.sub.1 +V.sub.2) after it.
(7) Similarly, development is made for the third and fourth colors.
(8) If the color toners are all superimposed, then they are all transferred to the image carrying material at one time.
However, the photoreceptor body color superimposition type has the following disadvantages.
The color toner, for example, the yellow toner, developed on the photoreceptor body is charged to negative. This causes the formed toner layer to have a potential of the same polarity as the photoreceptor body. The potential is in proportion to the amount of the toner. With this potential, any of the developed surface portions has the toner layer potential added thereto.
Therefore, if the whole surface of the photoreceptor body is charged again to develop the second color toner, for example, the magenta toner, the surface potential of the photoreceptor is increased at the portions having the yellow toner developed. If the magenta toner is exposed at developing portions, in turn, the surface potential of the photoreceptor body is decreased. As the portions having the preceding yellow toner have higher potential, however, the exposed portions have higher surface potential V.sub.L than the other portions. The potential V.sub.L is high with the amount of the yellow tone applied. That is, the difference of the developing bias contributing to development from V.sub.1 (V.sub.B -V.sub.L, hereinafter referred to as the development potential) becomes less. If the amount of the yellow toner applied is deviated, that of the following magenta toner applied also is deviated.
On the other hand, to make the superimposition development of the toners, it is optimum to use the non-contact development method, where only the toner to be developed is moved from the developing arrangement to the photoreceptor body while the developer does not contact with the photoreceptor body. However, the non-contact development method has an air layer formed between the developer layer and the photoreceptor body. This will not make the developer layer cause an opposing electrode effect so that the development electric field due to the development potential becomes weaker. This results in an edge effect, where the electric field is enhanced only on the boundary between the exposed portion and non-exposed portion.
As a result, sticking of the toners is concentrated on the edge portions. If the yellow toner is developed first and the magenta toner is developed second to obtain a red image, then the image is adversely affected around its outer edge by the excessive adhesion of the yellow toner, lowering the amount of the magenta toner applied. This is a disadvantageous enhancement of the yellow color in the image.